Field of the Invention
The present invention relates to a planar optical waveguide device used in, for example, optical fiber communication, and in particular, to a high-order polarization conversion device for performing polarization conversion, an optical waveguide device, and a DP-QPSK modulator.
Description of the Related Art
Currently, the amount of information transmitted by optical communication has been increasing with the spread of high-speed Internet access services, smartphones, and the like. In order to respond to such an increase in the amount of information, measures have been taken increase signal speed, increase the number of channels due to wavelength multiplexing communication, and the like. In particular, in the next generation of digital coherent transmission technology with transmission speeds of 100 Gbps (gigabits per second) for high-speed information communication, in order to double the amount of information per unit time, a polarization multiplexing scheme for carrying information in each of two polarized waves having electric fields perpendicular to each other is used. However, in modulation schemes for high-speed communication including the polarization multiplexing scheme, the structure of optical circuit components that form an optical modulator is complicated. For this reason, problems, such as increases in device size and cost, occur.
In addition, the required number of optical circuit components, such as a transceiver, increases with an increase in the amount of information transmitted by optical communication. Therefore, in order to increase the number of optical circuit components in a limited space, miniaturization and high-density integration of optical elements forming an optical circuit component are required.
In order to solve such problems, an optical circuit component (light modulator or the like) having a planar optical waveguide using silicon (silicon optical waveguide), which is advantageous in terms of easy processing, size reduction by integration, and cost reduction by mass production, has been studied and developed.
The silicon optical waveguide is a so-called large relative refractive index difference optical waveguide in which a silicon based material (Si, Si3N4, or the like) having a large refractive index is used for the core and a material (SiO2, air, Si3N4, or the like) having a large refractive index difference with respect to the core is used for a clad. If the relative refractive index difference is large, the confinement of light in the core is increased. Accordingly, since sharp bending becomes possible, it is suitable for the miniaturization and high-density integration of optical elements.
However, the polarization multiplexing in the planar optical waveguide has the following problems. In general, the planar optical waveguide has a shape in which a width direction parallel to the substrate and a height direction perpendicular to the substrate are asymmetric. For this reason, in two types of polarization modes of a mode substantially having only an electric field component in the width direction (hereinafter, referred to as a TE mode) and a mode substantially having only an electric field component in the height direction (hereinafter, referred to as a TM mode), the characteristics, such as an effective refractive index, are different. In these modes, a fundamental TE mode (TE0) and a fundamental TM mode (TM0) are used in many cases. Here, TE0 refers to a mode having the largest effective refractive index of the TE modes. In addition, TM0 refers to a mode having the largest effective refractive index of the TM modes.
It is difficult to perform an optical modulation operation for these modes having different characteristics with a single planar optical waveguide device. When a planar optical waveguide device optimized for each mode is required, a lot of effort is required in terms of the development of the planar optical waveguide devices.
As a method for solving this problem, a method can be mentioned in which TE0 is used as light incident on a desired planar optical waveguide device optimized for TE0 and the output is polarization-converted to TM0. The polarization conversion herein indicates a conversion from TE0 to TM0 or a conversion from TM0 to TE0. In order to perform the operation described above, a planar optical waveguide device for performing polarization conversion on the substrate is required.
As a technique of performing such polarization conversion on the substrate, there is a method of converting TE0 to a high-order TE mode (TE1) and then converting TE1 to TM0. Here, TE1 indicates a TE mode having the second largest effective refractive index. Since TE1 has an electric field component in the same direction as TE0, the conversion can be realized by using a directional coupler that can be manufactured through a simple process, such as arranging rectangular optical waveguides in parallel. Therefore, if the device that converts TE1 to TM0 is realized, it is possible to perform polarization conversion through TE1.
In general, since the silicon optical waveguide has a large birefringence, the silicon optical waveguide has strong polarization dependence. For example, when TE0 and TM0 are input to an optical element, the characteristics of the optical element are significantly different. In order to solve this problem, a polarization diversity scheme to input the same mode to the optical element using a polarization conversion device for converting TM0 into TE0 (or vice versa) is used. Therefore, in order to perform miniaturization and high-density integration of optical elements, a small polarization conversion device is essential.
As a technique for the polarization conversion device using a silicon optical waveguide, a method of converting TE0 to TE1 and then converting TE1 to TM0 has been proposed.
As a technique for performing polarization conversion using such conversion between TE1 and TM0 (hereinafter, referred to as high-order polarization conversion) on the planar optical waveguide, Daoxin Dai and John E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires,” Optics Express, Vol. 19, No. 11, pp. 10940 (2011) (hereinafter, referred to as NPL 1) can be mentioned.
An example thereof is shown in FIGS. 2(a) and 2(b) in NPL 1.
The optical waveguide device disclosed in NPL 1 is configured to include a directional coupler portion (coupling portion) and a tapered optical waveguide portion (tapered portion), and has a structure in which the emission end of the coupling portion is connected to the tapered portion. The coupling portion converts TE0 to TE1, and the tapered portion is a planar optical waveguide device that converts TE1 to TM0. The sectional distribution perpendicular to the guiding direction of the refractive index of the optical waveguide used in the two portions is shown in the graphs of FIGS. 1(a) and 1(c) in NPL 1. A rectangular portion called a core, a lower clad that is located below the core and has a lower refractive index than the core, and an upper clad that has a lower refractive index than the core and covers a different core from the lower clad are shown in these diagrams.
In FIGS. 1(a) and 1(c) in NPL 1, the graph of the effective refractive index with respect to the core width is shown. The core is formed of Si and has a refractive index of 3.455, the lower clad is formed of SiO2 and has a refractive index of 1.445, and the upper clad is formed of air (refractive index is 1.0) or Si3N4 (refractive index is 2.0). The height of the core is set to 220 nm.
In addition, the graph of the effective refractive index of the optical waveguide having a vertically symmetric refractive index sectional shape, in which the upper clad and the lower clad have the same refractive index is shown in FIG. 1(b) in NPL 1.
As can be seen from these diagrams, when the refractive index section has a vertically asymmetric refractive index sectional structure, in the graph of a change in the effective refractive index of each mode with respect to a change in the width direction, points of degenerate TE1 and TM0 are separated from each other in a waveguide having a vertically symmetric refractive index sectional structure.
For example, in the graph of FIG. 1(a) in NPL 1, near the waveguide width of 0.7 μm, as the waveguide width increases, a change from TM0 (fundamental TM mode) to TE1 (high-order TE mode) is shown in a mode having the second highest effective refractive index, and a change from TE1 (high-order TE mode) to TM0 (fundamental TM mode) is shown in a mode having the third highest effective refractive index. Accordingly, since TE1 and TM0 are continuously connected to each other in the effective refractive index curve shape, it is possible to perform high-order polarization conversion with low loss by gently changing the waveguide width. Using this phenomenon, high-order polarization conversion is performed by forming the tapered portion in the polarization conversion device described above in a tapered structure in which the waveguide width is gently changed in the range of conversion from TE1 to TM0.
Daoxin Dai, Yongbo Tang, and John E Bowers, “Mode conversion in tapered submicron silicon ridge optical waveguides,” Optics Express, Vol. 20, No. 12, pp. 13425-13439 (2012) (hereinafter, referred to as NPL 2) discloses performing high-order polarization conversion by making the sectional structure of the core vertically asymmetric using the same material (SiO2) for the upper and lower clads.
NPL 2 discloses a high-order polarization conversion device, in which one end of the section of an input and output portion has a sectional structure of a rib waveguide and the other end has a sectional structure of a rectangular waveguide, in FIG. 11 and the like.
NPL 1 discloses that materials having different refractive indices are required for the upper clad and the lower clad in the tapered portion for performing high-order polarization conversion. When using such new materials, an extra process occurs, or materials that are not used in other optical waveguide portions originally are required. Therefore, this is disadvantageous in terms of efficiency or cost. If different materials are used for the upper clad and the lower clad, distortion occurs due to a difference in linear expansion coefficients or the like. This lowers the yield. In addition, it is also possible to mention a method in which the lower clad is formed of a material used for the optical waveguide and the material of the upper clad is air. However, since the optical waveguide is exposed during the manufacturing process, the characteristics are degraded due to adhesion of foreign matter. As a result, the yield is reduced.
In the structure disclosed in NPL 2, the clad region in the width direction of the rib waveguide is narrow. Accordingly, since the confinement of light in the width direction is weak, large loss may occur in a bent waveguide portion due to a steep bending radius. For this reason, when using the rib waveguide, it is necessary to increase the bending radius (several tens to several hundreds of micrometers), and it is difficult to realize high-density integration in the structure disclosed in NPL 2. That is, in order to realize the high-density integration of optical elements in an optical circuit component, the optical elements need to be connected to each other by a rectangular waveguide that is sufficiently covered by the clad in the width direction. In addition, even if a structure for conversion from the rib waveguide to the rectangular waveguide is combined at the end of the rib waveguide disclosed in NPL 2, a rectangle-rib conversion portion is required. Therefore, it is difficult to miniaturize the optical element.
The present invention has been made in view of the aforementioned situation, and it is an object of the present invention to provide a high-order polarization conversion device and an optical waveguide device capable of performing polarization conversion between TE1 and TM0 even if the upper clad and the lower clad have different refractive indices, and to provide a high-order polarization conversion device and an optical waveguide device in which both miniaturization and high-density integration are possible.